Field monitoring of an unsaturated saprolitic hillslope

To advance the understanding of the rainfall-induced landslide triggering mechanism, a comprehensive field monitoring programme was implemented in a saprolitic hillslope in Hong Kong. The instrumentation covered the measurements of the two stress-state variables (i.e., net normal stress and matric suction) and their effects. The monitoring results, including pore-water pressure (PWP), volumetric water content, subsurface total horizontal stress, horizontal displacement, and rainfall intensity, are reported. Most instruments recorded reliable and good quality data, which have strong correlation among each other. The site-specific infiltration and deformation characteristics of the hillslope subjected to heavy rainstorms are investigated. Shallow transient perched groundwater tables were believed to be developed at colluvial deposits on the top 3 m, where PWPs up to 20 kPa were typically measured. The main groundwater table probably rose by 6 m when the hillslope was subjected to rainfall intensity of 133.5 mm/h. It is possible that cross-slope groundwater flowed along a shallow, dipped decomposed rock stratum at the central portion of the landslide body. Besides, two distinct types of slope movements were generally observed, namely the cantilever and the deep-seated mode. Rupture surfaces have possibly been developed at 5 m below ground or deeper, resulting in a multiple translational- and rotational-slide type of failure.